The loss of data stored in a storage will be a major obstacle. For this reason, the storage is designed to copy data that is stored in a volatile cache memory but is not stored in a disk drive or other storage media, to a nonvolatile memory during a power outage or other electrical emergency, and to return the data from the nonvolatile memory to the cache memory after the power is recovered. Such a technique is disclosed, for example, in Patent Literature 1.